Sports concussion is a traumatic brain injury and common injury in sports that results in symptoms and impaired function. The goals of medical management for sports concussion are to allow the brain to recover and protect it from further trauma. Guidelines for return-to-play require symptom resolution (at rest and with activity) and a return to pre-concussion function. However, current return-to-play criteria may not guarantee the resolution of all post-concussion impairments. Our preliminary data and other literature findings suggest that sports concussion might alter neuromuscular function and increase the risk for lower extremity musculoskeletal injury. Neuromuscular function needs to be examined more closely at the time of return to play after sports concussion to determine if a residual impairment exists that could increase risk for lower extremity injury. The objective of this application is to examine neuromuscular function of the thigh muscles following sports concussion. The central hypothesis is that athletes with concussion will have altered neuromuscular function of the thigh muscles at the time of return to play and 30 days later when compared to athletes without concussion. This is a prospective cohort study of male and female athletes with concussion (n = 36) matched to athletes without concussion (n = 36). Athletes with concussion will be tested within 10 days of receiving clearance to return to play and 30 days later. Athletes without concussion will be tested at 2 time points spaced 30 days apart. The specific aims of this study are to compare the foci of brain activation during a quadriceps muscle contraction stimulus (Specific Aim 1), voluntary activation and strength of the quadriceps muscle (Specific Aim 2), and the independent and combined effects of concussion and cognitive challenge on knee stiffness during a jump land task (Specific Aim 3) in athletes with or without a concussion. Functional magnetic resonance imaging (fMRI) will be conducted at the first testing time point only, whereas maximal voluntary isometric contraction with interpolated electrical doublet testing and movement analysis will be conducted at both testing time points. Additional testing includes neurocognitive testing, isokinetic strength testing, and surface electromyography (during movement analysis). The long-term goal of this research line is to understand the effect of sports concussion on the neuromusculoskeletal system and integrate that knowledge in post-concussion rehabilitation and return-to-play protocols. Current sports concussion medical management does not account for neuromuscular impairments or lower extremity injury risk. Findings from this study could have transformative implications for medical practice, patient function, and medical care costs. This application is based on a unique conceptual mode, focuses on an under-explored area of sports concussion research, and includes novel methods for assessing neuromuscular function with fMRI and movement analysis.